Apparatus for aligning a first article relative to a second article

ABSTRACT

A method and apparatus is described for aligning a first article relative to a second article, for example for aligning a nanoimprint template with a semiconductor wafer. The method comprises the steps of: providing said second article with at least one flexible structure fixed relative thereto at least one point, providing a first article having at least one surface relief marking thereon, providing a detector for measuring an interaction of the flexible structure with the surface relief marking and generating detector signals relating to said interaction, identifying with the help of the detector signals the position of the flexible structure and thus of the second article with respect to the surface relief marking and generating relative movement between the first and second articles to achieve a desired alignment between the first and second articles defined by the surface relief marking. In this method and apparatus the flexible structure is brought into contact with the surface relief marking.

FIELD OF THE INVENTION

The present invention relates to a method of aligning a first articlerelative to a second article and to an apparatus for aligning a firstarticle relative to a second article and has particular reference to amethod and to an apparatus for the high precision alignment of twoarticles, e.g. to a positional accuracy better than ten nanometres andpreferably better than one nanometre.

BACKGROUND OF THE INVENTION

Currently alignment techniques are known, e.g. in the field ofsemiconductor manufacture, where an alignment with a positional accuracyof about 100 nm can be achieved with optical techniques. However, it isvery difficult to achieve an alignment accuracy significantly betterthan 10 nm because of the diffraction limit which applies to theresolution of optical alignment techniques.

There are some semiconductor manufacturing processes involvingnano-structures which would benefit from an alignment accuracy of betterthan 10 nm and preferably of around 1 nm which cannot currently beachieved, or can only be achieved with significant technical complexity.One such application is the manufacture of semiconductor circuits usingtemplates and curable resins to define the circuit patterns which are tobe realized. Such manufacturing processes are summarized below.

There are also other technical fields in which a high alignment accuracyis either currently desirable or which could benefit from such highalignment accuracies, if suitable methods and apparatus for achievingalignment accuracies of tens of nm or better were available in aproduction environment. For example, there is increasing interest innano-electronics, molecular electronics, single electron devices, ormicrofluidic devices which could be made significantly smaller if higheralignment accuracies could be achieved.

Similarly applications for high precision alignment apparatus andmethods are conceivable in the biological or chemical fields. Forexample one could conceive a holder for biological or chemical usehaving an array of regularly (or irregularly) positioned recesses on ananometer scale, each containing a sample or a reagent which has to bebrought together with high positional accuracy with a carrier havingreagents or samples positioned on a complementary array of projectionswhich have to engage in the aforementioned recesses.

In nanoimprint lithography in which a negative three dimensional patternprovided on a template, for example of fused silica, is transferred to athin layer of silicon containing monomer on a semiconductor orinsulating substrate which is subsequently polymerized by UVillumination to form a hard positive pattern of the cured polymer on thesurface of the substrate. In this technique the cured polymer issubsequently etched to remove a residual layer of polymer between raisedfeatures of the pattern and to reach the substrate material at thesepositions. Thereafter, the substrate may be etched further to producedepressions in the substrate material, and increase the aspect ratio ofthe raised features relative to the depressions, i.e. the depth of thedepressions relative to the raised features. Then the residual polymercan be stripped from the substrate and one or more layers ofsemiconducting or insulating or conducting material deposited on thesubstrate. Following this, and appropriate polishing of the surface ofthe substrate, an organic planarization layer is deposited on thesubstrate and the process is then repeated using a different templateand a new layer of UV-curable imprint solution. This process is thenrepeated for further templates, for example frequently using twenty orthirty different templates to produce the finished semiconductorcircuit. This process, known e.g. under the name S-FIL™, a registeredtrade mark of Molecular Imprints Inc., is discussed, together with otherlithographic processes, in more detail in an appendix to the presentapplication.

The templates used are for example available to order from firms such asDuPont Photomasks and Photronics, who currently take orders for S-FILtemplates down to 100 nm feature sizes. Only the template fabricationprocess, typically accomplished with an e-beam writer, limits theresolution of the features. Features as small as 20 nm have been made todate that exceed the present requirements specified in the InternationalTechnology Roadmap Semiconductors (ITRS). With this background in mindit will be appreciated that, although there is generally no criticalalignment problem with applying the first imprint to a substrate using afirst template, any subsequent template requires critical alignment withthe pattern determined by the first and succeeding templates if thesemiconductor circuit is to have any chance of operating as desired. Thepresent invention provides such a tool.

In known processes of manufacturing semiconductors surface reliefmarkings are regularly applied to semiconductor wafers to enablealignment of a series of masks or imprint templates with the wafer. Theyare however recognized and used for alignment by optical systems which,as explained above, have a diffraction limited resolution limit of about100 nm. The same surface relief markings can be used for the purposes ofthe present invention. However, it can be preferable to make themsmaller and for to position them with a smaller pitch. The surfacerelief markings can also be made with special topographies which enablethem and/or their position to be detected more accurately and/orreliably.

One proposal for achieving high accuracies in the alignment of apatterned mold (a first article) with a substrate (a second article)suitable for the manufacture of a semiconductor structure by an imprintprocess is described in the published international patent applicationWO 02/077716. That document describes a lithographic method whichcomprises aligning a patterned mold with respect to an alignment markdisposed on the substrate. The detection process is based uponinteraction of a scanning probe with the alignment mark. The alignmentmark can be formed by the edges of relief features provided on thesubstrate outside of the area to be patterned.

In the system described in WO 02/077716 an optical alignment system isfirst used to approximately align the patterned mold with the substrateand the precise alignment of the patterned mold with the substrate isthen effected by a scanning probe alignment system either realized as ascanning tunnel microscope scanning assembly, in which the positions ofthe probes and thus of the patterned mold are controlled based ontunneling current information, or implemented as an atomic forcemicroscope scanning assembly, in which the positions of probes arecontrolled based upon a force (e.g. an atomic force, an electrostaticforce or a magnetic force) that is generated between the probes and oneor more alignment marks carried on the substrate.

More specifically the scanning system is configured to move a scanninghead, which carries the patterned mold and the probes, precisely in aplane, the x-y plane, that is parallel to the support surface of astationary block carrying the substrate. The scanning system is alsoconfigured to move the scanning head precisely in the z-direction whichis orthogonal to the support surface of the stationary block. It isstated that in one embodiment the scanning head may be moved verticallyby a z-axis scan actuator and horizontally by a separate x-y axis scanactuator.

The actuators can be implemented as planar electrostatic actuators andcan both be carried on the scanning head. After scanning the alignmentmark in the x-y plane using the x-y scan actuator the precise positionof the alignment mark relative to the patterned mold can be determinedand the x-y actuators used to move the patterned mold into the desiredalignment with the alignment marks and thus the substrate for thepatterning process. The z-actuator can then be used to move thepatterned mold vertically to impress the pattern thereon into a moldablefilm provided on the substrate. It is also stated that the probes can beretracted after alignment prior to the movement in the z-direction.However, it is not explained how this can be done.

The problem with the scanning system described in WO 02/077716 is thatthe detection of the alignment marks through scanning movement of theprobes takes a relatively long time. This means that the manufacturingprocess for the semiconductor structure, which can involve the use ofmany imprint steps, with processing steps following each imprint stepthus requiring repeated realignment, takes a relatively long time, whichis undesirable. It should be appreciated that this applies irrespectiveof whether the scanning system is realized as a scanning tunnelmicroscope or is based on an atomic force microscope scanning assemblyin which the positions of the probes are controlled based upon a force,such as an atomic force, an electrostatic force or a magnetic force. Asthose skilled in the art of atomic force microscopes will know these areall time-consuming non-contact measurement techniques. U.S. Pat. No.5,317,141 describes a similar system to WO 02/077716. The principaldifference is that the US patent is concerned with the alignment of amask for X-ray lithography with a wafer, for subsequent patterning ofthe wafer using x-ray beams directed through the X-ray mask. Again ascanning probe microscope is used, e.g. in the form of an atomic forcemicroscope which functions by scanning a fine-tipped probe over thesurface of an alignment mark on the wafer or substrate. Morespecifically, a voltage difference is applied between the probe and thealignment mark and the tunneling current which results when the probe isa small distance from the surface is detected. For this system thealignment mark must have a conductive surface.

This is again a non-contact measurement. The detection of the tunnelingcurrent during scanning of the probe over the alignment mark is effectedby a piezoelectric block carrying the probe. Control voltages can beapplied to electrodes on the piezoelectric block to first energize it tomove the probe in the z-direction to detect a tunneling current as theprobe approaches the surface of the alignment mark. Thereafter furthercontrol voltages can be applied to appropriately positioned electrodeson the piezoelectric block to produce scanning movement of the probe inthe x- and y-directions. The variation in the tunneling current and thusthe topology of the alignment mark can then be determined during thescanning movements. This allows the position of the sensing headrelative to the alignment mark to be determined with high accuracy.Because of the restriction involving the need for the alignment mark tohave a conductive surface the US patent also discloses a second systemin which a contact arm touches the surface of the alignment mark and thesensing tip is carried by the piezoelectric block at a small distanceabove the arm. The arm is electrically conductive so that the tunnelingcurrent can be measured between the arm and the tip of the probe whichis spaced from the arm. The distance between the arm and the tip of theprobe varies as the probe and arm are scanned over the surface of thealignment mark. Again the scanning of the alignment mark is relativelyslow.

For the sake of completeness reference should also be made to twofurther documents which refer generally to atomic force microscopy. Thefirst is DE-A-103 03 040 which describes a non-contact mode detectorincorporated in a cantilever. This non-contact mode detector is alsodescribed, together with other non-contact detectors, in the paper:

Micromachined atomic force microscopy sensor with integratedpiezoresistive sensor and thermal bimorph actuator for high speednon-contact mode atomic force microscopy phase imaging in highereigenmodes by R. Pedrak, Tzv. Ivanov, K. Ivanova, T Gotszalk, N.Abedinov, I. W. Rangelow, K. Edinger, E. Tomerov, T. Schenkel and P.Hudek in J. Vac. Sci. Technol. B 21(6) November/December 2003 pages 3102to 3107. More specifically the above referenced article describesmicroprobes for non-contact scanning force microscopy, more specificallytapping mode atomic force microscopy. In this arrangement a cantilevercarrying a tip is excited to oscillate close to its resonance. Thetopography information is collected from the phase lag between vibrationexcitation and response of the cantilever deflection sensor.

In one embodiment described in the above referenced article thecantilever is realized as a bimorph actuator involving an aluminiumlayer on a silicon dioxide cantilever. The aluminium layer can be heatedwith an oscillating current to produce oscillatory bending deflection ofthe cantilever due to differential thermal expansion. In one embodimentdescribed in the paper the cantilever is configured to include apiezoresistive detector realized in the form of a Wheatstone bridge.

The production of probes for atomic force microscopy, including a probewhich utilizes a piezoresistive Wheatstone bridge is also described inthe document SPIE Vol 2879/pages 56 to 64, being a paper presented at aconference in Texas on Oct. 14th to 15th 1996. A piezoresistive detectorincorporated in a cantilever is also described in U.S. Pat. No.5,444,244.

SUMMARY OF THE INVENTION

It is therefore a principle object of the present invention to provide amethod and an apparatus for accurately aligning a first article with asecond article with a positional accuracy better than 100 nm, preferablybetter than 10 nm and in particular approaching 1 nm or better at asignificantly higher speed than is possible in the prior art, thusspeeding up production processes such as the manufacture ofsemiconductor substrates by imprint lithography.

In order to satisfy this object there is provided a method of aligning afirst article with a second article comprising the steps of:

-   -   providing said second article with at least one flexible        structure fixed relative thereto at least one point,    -   providing a first article having at least one surface relief        marking,    -   providing a detector for measuring an interaction of the        flexible structure with the surface relief marking and    -   generating detector signals relating to said interaction,    -   identifying with the help of the detector signals the position        of the flexible structure and thus of the second article with        respect to the surface relief marking and generating relative        movement between the first and second articles to achieve a        desired alignment between the first and second articles defined        by the surface relief marking, the method being characterised in        that said detector detects deflection of said flexible structure        by a tip of the flexible structure, e.g. a cantilever tip,        touching the surface, the detector preferably being selected        from the group comprising: a reflecting surface provided on said        flexible structure and an associated optical detection system        for measuring said deflection at said reflecting surface, a        piezoresistive structure incorporated at said flexible        structure, a piezoelectric structure incorporated at said        flexible structure, a capacitive detector for detecting        deflection of said flexible structure, an inductive detector for        detecting deflection of said flexible structure and a surface        acoustic wave structure incorporated at said flexible structure.

Also according to the present invention there is provided an apparatusfor aligning a first article with a second article, said first articlehaving at least one surface relief marking and said second articlehaving at least one flexible structure, means for positioning one ofsaid first and second articles adjacent to the other one with provisionfor at least restricted relative movement between said first and secondarticles, a detector for detecting deflection of said at least oneflexible structure due to an interaction with said at least one surfacerelief marking and providing detecting signals relating to saidinteraction, a memory containing stored information related to atopology of said surface relief marking, means for comparing saiddetection signals with said stored information to generate positioncontrol signals relating to a desired alignment and steering means forsteering movement of at least one of said first and second articlesrelative to the other to achieve said desired alignment the apparatusbeing characterised in that said detector is adapted to detectdeflection of said flexible structure by a tip of the flexiblestructure, e.g. a cantilever tip, touching the surface, the detectorpreferably being selected from the group comprising: a reflectingsurface provided on said flexible structure and an associated opticaldetection system for measuring said deflection at said reflectingsurface, a piezoresistive structure incorporated at said flexiblestructure, a piezoelectric structure incorporated at said flexiblestructure, a capacitive detector for detecting deflection of saidflexible structure, an inductive detector for detecting deflection ofsaid flexible structure and a surface acoustic wave structureincorporated at said flexible structure.

A method and an apparatus of the above kind are very advantageous. Sincethe tip of the flexible structure actually touches the alignment mark,or at least prominent edges of it, scanning movement to detect thealignment mark can be carried out very fast, thus significantly reducingthe processing time required to detect the position of the alignmentmark and thus to align the first article with the second for the imprintprocess. There is no need to vary the distance between the secondarticle carrying the flexible structure and the first article once theflexible structure has touched the surface of the first article, i.e.the alignment mark provided thereon. The alignment mark is preferablyprovided with sharp edges which enhance the detection signals resultingfrom deflection of the flexible structure as it is moved across thealignment mark, which can be done relatively rapidly.

In a particularly preferred embodiment of the method and the apparatusthe flexible structure is provided as a bimorph structure enabling it tobe heated electrically resulting in differential expansion of two layersof the flexible structure (which may be a cantilever) and thus bendingdeflection which allows the tip on the flexible structure to beretracted during the imprint patterning movement so that the neither theflexible structure nor the tip is damaged during the imprint process.This retraction movement can be realized almost instantaneouslyfollowing the designed alignment having been achieved so thatessentially no time is lost by this operation. In principle it is onlynecessary to provide a single surface relief marking on the firstarticle or substrate, i.e. a surface relief marking at one position onthe first article or substrate, providing this marking has sufficientdifferently aligned features that it can be recognized by interactionwith the flexible structure and allow precise alignment to be achieved.Generally it is however simpler to provide a plurality of surface reliefmarkings on the first article or substrate, i.e. surface relief markingsat different locations on the first article or substrate because thesecan be made more simply and recognized more simply. The markings could,for example, then comprise a plurality of raised bars or bar-likedepressions at one point on the margin of an article or wafer and asecond, like, set of surface relief markings at a different angularorientation (e.g. at 90° to the first set) at a different position onthe margin of the article or wafer (e.g. displaced around an axis of thearticle or substrate by 90° relative to the first set).

If a plurality of surface relief markings are provided then it isgenerally convenient if a like plurality of flexible structures isprovided on the second article, which are placed in juxtaposition to therespective surface relief markings.

The surface markings are typically surface relief markings which areintegrally formed with said first article. For example, in the case of asemiconductor wafer, the surface relief markings can be formed on thewafer by electron beam writing or by any suitable lithography process.

In principle the surface relief marking can be a natural feature of saidfirst article, e.g. surface features such as corrugations or pyramidsformed by self organization of the surface or natural structures orfaults. Alternatively it can be an artificial feature formed on orbonded to the first article. In similar fashion the or each flexiblestructure can be integrally formed with said second article or can bedistinct from said second article, but physically connected thereto.

The flexible structure can be selected from the group comprising: acantilever, a flexible bridge supported at two points and a flexiblemembrane supported at a plurality of points. The most convenient designis a cantilever, for example a cantilever as is used in an atomic forcemicroscope. The detector is typically adapted to detect a deflection ofsaid flexible structure which arises as a result of the interaction.

As mentioned, the first article can be a semiconductor wafer or aninsulating substrate, or a substrate with a partially completedsemiconductor structure formed thereon. It can also be a glass, metal orplastic article, a biological sample carrier or a chemical samplecarrier or can take some other form. It can, for example, also be aplanar or contoured surface of a three dimensional body.

The second article can be a template for nanoimprint lithography asdescribed above or a biological sample dispenser, a chemical sampledispenser, a biological sample readout device or a chemical samplereadout device or some other article.

A means will generally be provided for steering movement of said secondarticle relative to said first article in order to achieve the requisitealignment. Thus, either the first article can be held stationary and thesecond article moved relative to it, or vice versa. Alternatively, thefirst and second articles can both be moved simultaneously.

More specifically, said steering means can be selected, withoutlimitation, from the group comprising piezoelectric actuators, thermalactuators, electromagnetic actuators, and oscillatory actuators. With anoscillatory actuator the second article would oscillate relative to thefirst and the second article could be moved into contact with the firstat the correct position of alignment reached at some point during theoscillatory movement, thus arresting the oscillatory movement, at leasttemporarily; for example, for the duration of an S-FIL imprint step.

It is not essential to provide relative movement of the second articlerelative to the first in order to find the correct alignment, although arelative movement of this kind will generally be provided in order toenable the flexible structure to interact with the surface reliefmarking. It would for example be possible to realize the flexiblestructure as one moveable in two distinct planes, for example in twoplanes orthogonal to one another. In this case the flexibility in oneplane could be used to sense the distance to the surface, i.e. to findwhether a step or edge in the surface relief marking is present, whereasthe movement in the second plane, which would be a steered movement,caused for example by bending of a cantilever when subjected to heating,is used to traverse the surface relief marking in a direction generallyparallel to the surface of the first article. Thus a cantilever asdescribed in the above referenced article could be provided with bimorphproperties on two orthogonal surfaces of the cantilever.

BRIEF DESCRIPTION OF THE DRAWINGS

The above described methods can be repeated using a third article orfurther articles provided with a flexible structure and cooperating withsaid surface relief marking provided on said first article. Furtherpreferred embodiments of the invention are described in the claims andin the following description of preferred embodiments given withreference to the accompanying drawings in which:

FIG. 1 shows a schematic diagram of an apparatus in accordance with thepresent invention,

FIG. 2 shows a plan view of a semiconductor wafer having various imprintareas each with its own surface relief markings for use in the presentinvention,

FIG. 3 shows a silicon structure forming a starting point formanufacturing a second article in accordance with the present invention,

FIG. 4 shows the silicon structure of FIG. 3 after formation of atemplate thereon,

FIG. 5 shows a step for the protection of the template of FIG. 4,

FIG. 6 shows a first variant of the silicon structure of FIG. 4 afterthe formation of the flexible structures thereon,

FIG. 7 shows a variant of the silicon structure of FIG. 6,

FIG. 8 shows a further variant of the silicon structure of FIG. 6

FIG. 9 shows an optical detector used in conjunction with a structureresembling that of FIG. 6,

FIG. 10 shows a variant of the structure of FIG. 9,

FIG. 11 shows schematically and not to scale a plan view on a flexiblestructure such as is illustrated in the form of a cantilever in any ofthe FIGS., 6, 7 and 8,

FIG. 12 shows in FIG. 12A a detailed cross-section through a typicalcantilever useful in practice in a first deflected position and in FIG.12B the same cantilever in a second retracted position,

FIG. 13 shows in schematic form an alternative flexible structure in theform of a flexible bridge formed in a portion of a wafer and supportedat two points, with FIG. 13A showing the bridge in a longitudinalsection in accordance with the section plane XIIIA-XIIIA of FIG. 13B andFIG. 13B showing the bridge in plan view in accordance with the arrowXIIIB of FIG. 13 A,

FIG. 14 shows an alternative flexible structure in the form of aflexible membrane supported at three points, with FIG. 14A showing in aschematic representation, not to scale, a part of a wafer provided withthe membrane in plan view and FIGS. 14B and 14C showing cross sectionson the section planes XIVB-XIVB and XIVC-XIVC respectively,

FIG. 15 shows an alternative flexible structure in the form of aflexible membrane supported at four points, in representations FIGS.15A, 15B and 15C corresponding to those of FIGS. 14A, 14B and 14C,

FIG. 16 illustrates the use of the invention in the chemical orbiological field,

FIG. 17 illustrates one embodiment of a flexible structure used as acontact detector in which detection of the deflection of the flexiblestructure at the edges of an alignment mark is effected by measuring thevoltage drop on a piezoresistor formed on the flexible structure,

FIG. 18 illustrates another embodiment of a flexible structure used as acontact detector in which detection of the deflection of the flexiblestructure at the edges of an alignment mark is effected by measuring thevoltage signal from a piezoelectric layer formed on the flexiblestructure,

FIG. 19 illustrates a further embodiment of a flexible structure used asa contact detector in which detection of the deflection of the flexiblestructure at the edges of an alignment mark is effected by measuring thechange in capacitance of a capacitor formed on the flexible structure,

FIG. 20 illustrates yet another embodiment of a flexible structure usedas a contact detector in which detection of the deflection of theflexible structure at the edges of an alignment mark is effected bymeasuring the voltage induced in one coil formed on the flexiblestructure by a second coil formed thereon,

FIG. 21 illustrates one embodiment of a flexible structure used as acontact detector in which detection of the deflection of the flexiblestructure at the edges of an alignment mark is effected by a surfacewave transition at an inter-digital structure formed on the flexiblestructure,

FIG. 22 illustrates an embodiment of a flexible structure similar tothat of FIG. 9 used as a contact detector in which detection of thedeflection of the flexible structure at the edges of an alignment markis effected by utilizing a laser beam reflected at the flexiblestructure and a beam position detector,

FIG. 23 illustrates the deflection of the flexible structure of FIG. 22in a perspective view and

FIG. 24 shows how the beam position changes as a function of thedeflection of the flexible structure of FIGS. 22 and 23.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1 there can be seen a first article 10, here inthe form of a semiconductor wafer, which is rigidly mounted on a fixedtable 12. Positioned above the first article 10 and facing it is anadjacent second article, here in the form of a silicon wafer 14 carryinga template 16 which is to be used in a nanoimprint lithography stepusing the S-FIL process to pattern an imprint resist layer 18 depositedon an imprint area 20 of the semiconductor wafer forming the firstarticle 10.

In this embodiment the first article 10 has first and second surfacerelief markings 22 and 24. The surface relief marking 22 is illustratedas a row of triangular bars provided locally on the surface of the firstarticle 10 and seen end on in this drawing. The second surface reliefmarking 24 is essentially identical to the surface relief marking 22 butis rotated through 90° relative to the marking 22 thus the bars are seenside on in the marking 24. It will be appreciated that the markings 22and 24 have a fixed position relative to the imprint area 20 and anycircuit pattern provided thereon. Accordingly, if appropriate referencemarks on the second article 14, which have a fixed position relative tothe template 16, are accurately aligned with the markings 22 and 24then, assuming the template is correctly positioned with respect to thereference marks, the template is accurately aligned with the imprintarea 20 and any circuit pattern provided thereon.

In the present embodiment the reference marks on the second article 14take the of first and second flexible structures 26, 28 which are eachrealized as a cantilever beam. The cantilever beam 26 forming the firstflexible structure is seen side on in FIG. 1 and carries a fine tip 30which is formed in the manner of a fine tip typically used in atomicforce microscopy (i.e. a tip which is preferably just one atom wide atthe point) and is able to touch and scan across the surface reliefmarking 22 in the x-direction to detect the positions of the edges ofthe bars and indeed, if desired, also the heights of the bars formingthe marking 22. Such a scan enables the relative position of the secondarticle relative to the first article to be determined in one horizontaldirection in FIG. 1 (the x-direction).

The cantilever 28 shown at the right in FIG. 1 is seen end on and isprovided with a respective fine point 32 which is able to touch and scanthe second marking 24 in a direction perpendicular to the plane of thedrawing, i.e. in the y-direction, to determine its position relative tothe second marking and thus the alignment of the second article 14relative to the first in a second horizontal direction perpendicular tothe first. These two alignments thus ensure that the second article isaccurately aligned in juxtaposition relative to the first.

Moreover, since the heights of the bars of the markings 22 and 24 bear aknown relationship to the height of the imprint area the amplitude ofmovement required to achieve the desired imprint can also be determined.This is, however, not essential since the imprint movement could also beforce limited, i.e. carried out until a specific force is exerted on thefirst article 10 by the imprint stamp forming the second article 14.

It will be appreciated that the accuracy of the alignment is nowdependent on four things:

A) The alignment of the surface relief markings on the first articlewith the imprint area or the pattern provided thereon. This alignmentaccuracy can be achieved to a high level by patterning the surfacerelief markings at the same time the imprint area is generated on thesemiconductor wafer. Indeed it is not essential for the surface reliefmarkings to have an exactly predetermined position providing theirposition relative to the imprint area can be accurately determined,which is also possible using atomic force microscopy or a similarprocess. This also means that the surface relief markings could bemanufactured separately and subsequently bonded to the first article.

B) The alignment of the flexible structures 26, 28 on the second article14 relative to the template 16. Precisely the same considerations applyhere as given above in relation to the alignment of the surface reliefmarkings 22, 24 with respect to the first article 10.

C) The accuracy with which the surface relief markings 22, 24 can bemeasured by way of their interaction with the flexible structures 26,28. It is well established in the art of atomic force detectors thatmeasurement accuracies of 1 nm or better can be achieved in a variety ofways.

D) The accuracy with which the second article 14 can be moved relativeto the first 10. There are a variety of actuators which are known in thescientific community enabling movements with accuracies in the nanometrerange. Such actuators include piezoelectric actuators, thermal actuatorswhich operate by linear or differential thermal expansion,electromagnetic actuators, and oscillatory actuators.

In a practical embodiment the amplitudes of movement which can beachieved with actuators having accuracies in the nanometer range arerestricted and therefore the apparatus of the present invention willgenerally have a first means for approximate, coarse positioning of oneof said first and second articles 10, 14 adjacent to the other one aswell as provision for at least restricted, fine, relative movementbetween said first and second articles. The actuators for movement inthe range of several micrometers to one nanometer will be convenientlyprovided between the positioning means and the moved article, forexample the second article 14.

In the present embodiment the second article is shown supported by tworods 34, 36 in two guides 38 and 40 respectively. The guides areconnected to the table 12 as indicated by arms schematically indicatedby the lines 42, 44 and are thus fixed relative to the first article 10.They could also be connected to the first article 10. Only twopositioning/support means 34, 38 and 36, 40 are shown. In reality therehave to be as many positioning means as are required to allow basic(coarse) positioning of the second article relative to the first(generally in at least one plane parallel to the correspondingconfronting plane of the relevant oppositely disposed surface of thefirst article), which basic position may be checked using known opticalalignment techniques. That is to say the positioning means has to permitpositioning of the second article relative to the first in x- andy-directions, i.e. in the x-y plane. In addition either both coarse andfine positioning, or just fine positioning in the z-direction isnecessary for the imprint process depending on how the apparatus isdesigned.

E.g., if the second article, i.e. the patterned mold is placed only asmall distance above the first article, fine movement alone may benecessary to allow the imprint process to be carried out. However suchclose positioning may be difficult in practice, since the space forintroducing the substrate is restricted, unless it is itself movedtowards the patterned mold against a stop provided in the apparatusbefore alignment takes place. On the other hand if both coarse and finepositioning of the second article relative to the first may beappropriate to facilitate introduction of the substrate or wafer formingthe first article beneath the patterned mold.

The positioning means must also allow adequate space for the handling ofthe second article relative to the first.

In practice one stamp may be used to process a plurality of imprintareas 20 on one semiconductor wafer 10, see for example the plurality ofimprint areas 20 provided on the semiconductor wafer of FIG. 2. In thiscase the positioning means has to allow for a relative movement of thetemplate into coarse alignment with the individual imprint areas orgroups of imprint areas if a group of imprint areas, say four imprintareas is to be subjected to imprint lithography at one time, which wouldbe usual. Such positioning means are known per se they are used inconventional semiconductor processing.

In accordance with the present teaching actuators such as 46, 48 formingfine positioning means with accuracies in the nanometer range areincorporated between the coarse positioning means and the second articleso that once coarse positioning has been achieved the actuators can beenergized to carry out scanning movements of the second article 14 withthe flexible structures 26 and 28 relative to the surface reliefmarkings 22, 24. Once the precise position of the alignment marks hasbeen determined the actuators 46, 48 can then be used, i.e. controlledto accurately position the second article relative to the first withreference to the alignment marks. Alternatively the actuators can beused solely for fine alignment and the scanning movements of theflexible structures can be effected by separate actuators disposed onthe flexible structures or between the flexible structures and thesecond article to generate the required scanning movement.

FIG. 2 shows that the surface relief markings can also be provided onthe imprint areas 20 themselves and shows two surface relief markings22, 24 for each imprint area 20. In this case the surface reliefmarkings 22, 24 are provided in diametrically opposite corners of eachimprint area. This is not essential, they could for example be providedin all four corners. Only a single surface relief marking is necessaryif its topology is chosen so that accurate alignment can be achievedwith reference to that one marking. In such a case only a singleflexible structure is necessary. FIG. 2 also shows that a marking 22, 24divided into four quadrants with differently orientated bars in eachquadrant can be used for a surface relief marking. This is only one ofan infinite number of possibilities. It will however be understood thatit is relatively straightforward for a flexible structure configured inthe manner of a probe for an atomic force microscope to find the centerof the marking shown and to use its coordinates (or the coordinates of adifferent position of the quadrant) for the precise alignment of thesecond article 14 relative to the first 10. In fact it is preferable touse surface relief markings of the kind shown in FIG. 2 in theembodiment of FIG. 1 in which simplified markings were chosen tosimplify the explanation but would not readily allow accurate rotationalalignment around the z-axis. Such rotational alignment can be achievedwith the markings 22 and 24 shown in FIG. 2 (and also with a wholevariety of other markings).

Returning now to FIG. 1 the manner of operation can now be readilyunderstood. As mentioned above coarse positioning is first effected.This can be done manually or by computer control using the computer inthe form of the suitably programmed microprocessor 50 which obtainspositioning data from the memory 52 and can also make use of opticalpositioning information from auxiliary systems to steer the movement ofthe second article 14 relative to the first article 10. For this thesecond article 10 has to be mounted in a mechanism or carriage such isused in the fields of metrology or machine tools for movement of a probeor machining tool to a specific point in a coordinate system and toprovide the probe or machining tool with a desired spatial orientation.The positioning/support means schematically illustrated by the referencenumerals 34, 38 and 36, 40 in FIG. 1 can be realized in accordance withany known system for the accurate handling of probes or machining toolsor other items in a coordinate space. The optical positioninginformation can, for example, come from optical positioning systemsknown per se in the semiconductor field, for example in connection withthe existing S-FIL process. Also glass measuring rod or other scalesystems used in metrology and in machine tools can be used for basicpositioning of the second article relative to the first.

Once basic positioning has been achieved the basic positioning means islocked and actuators such as 46 and 48 are actuated to produce scanningmovement of the flexible structures 26, 32 to determine the preciserelative position of the second article relative to the first. Thereadout signals from the detectors associated with the flexiblestructures are amplified by a preamplifier 54 and fed to themicroprocessor 50. The microprocessor 50 analyses the signals from theactuators and compares them with reference information stored in thememory 52 relating to the topology of the surface relief markings 22 and24 and their positions relative to the imprint area 20.

Since the microprocessor 50 also has information concerning the relativepositions of the flexible structures relative to the template 16 it isable, from the result of the comparison, to issue positioning commandsto the actuators 46, 48 to accurately align the flexible structures withthe surface relief markings, for example to align the tips 30 and 32with the centers 60 and 62 of the markings 22, 24 of FIG. 2, and thus toaccurately align the second article 14 with the first 10 and thetemplate 16 with the imprint area 20 and any pattern already providedthereon.

Thus the microprocessor 50 generates, via the template stage positioncontrol 58 for the actuators, in this example piezomotors 46 and 48,steering signals for steering at least one of said first and secondarticles relative to the other to achieve said desired alignment. Thusthe template stage position control 58 forms steering means in the senseof the present invention. It could naturally also be integrated into themicroprocessor 50. The reference numeral 57 relates to a keyboard whichcan be used to input information into the system and the referencenumeral 59 indicates a screen which can provide user guidance menus anddisplay other information useful to the user. The items 50, 52, 54, 57,58 and 59 can all form part of a computer workstation or PC associatedwith the apparatus.

The signals obtained from the flexible members are described later withreference to FIGS. 17 to 24. After alignment of the first and secondarticles has been completed the imprint step is carried out by movementof the first article (i.e. the template or patterned mold) towards thesecond article (i.e. the substrate), or vice versa, by means of asuitable actuator acting on the second article (or the first article),so that the template enters into contact with the moldable film formedon the substrate. Once the imprint step has been completed the imprinttool forming the second object 14 can be moved to a different intendedimprint area 20 on the first object 10, as illustrated in FIG. 2 and thealignment process and imprint steps can be repeated there, using thesurface relief markings at the new imprint area 20. This process can berepeated as many times as required.

It is not essential to recheck the alignment for each imprint area,although this can be done and will frequently be done. It is alsoconsidered sufficient if the alignment is only checked for some of theimprint areas on the first object 10. For this possibility to berealized it is necessary to use a positioning/support system with highinherent accuracy. For example, such systems are known from the fieldsof metrology and precision engineering which use interferometry toachieve a high positioning accuracy which can approach the levelsrequired here. Thus, once the correct alignment has been found for oneimprint area it can be retained for one or more further imprintoperations. I.e. a type of step and repeat method can be used.

It should be noted that despite the existence of such highly precisepositioning systems they cannot be used to find the correct initialalignment, since the data for this, i.e. the correct initial positionfor the second object 14 relative to the first 10 has first to be foundusing the flexible structure approach of the present invention.

If adequately high precision positioning systems with a suitableamplitude of movement can be found, then they could be used both forbasic positioning and for the nano-range positioning. I.e. they could beused instead of the nano-range actuators such as 46, 48 for scanningmovement of the second object 14 and flexible structures 26, 28 relativeto the first object.

Turning now to FIGS. 3, 4, 5 and 6 a method will be described for themanufacture of a second article having flexible structures providedthereon.

FIG. 3 shows a starting wafer comprising a silicon wafer 70 having alayer of silicon dioxide (Si02) 72 followed by another layer ofepitaxial silicon 74 and a top layer of SiO2 (quartz) 76. Such wafersare commercially available as SOI wafers. Using such a wafer, a layer ofchromium is then deposited on the exposed surface of the upper layer 76,typically by a sputtering technique. A layer of photoresist is thenapplied to the chromium layer and is patterned using an electron beam tolocally expose the photoresist layer. The exposed photoresist layer isthen developed and this enables the chromium to be etched away locallyto expose the underlying layer of silicon dioxide 76. The remainingphotoresist covering the chromium that has not been etched away is thenremoved. Thereafter, the exposed layer of silicon dioxide 76 not coveredby the chromium can then be etched away using dry etching. The areasunderlying the remainder of the chromium layer are protected. Theremainder of the chromium layer is then removed. This leads to thestructure shown in FIG. 4 where the silicon dioxide layer has beenetched to form the desired template 16, the raised island shown at thetop in FIG. 4, and has been removed completely to the left and right ofthe template 16 to expose the underlying silicon material of the layer74. The template has now been finished and the entire wafer is coveredat the exposed upper surface with a layer 78 of silicon nitride(Si_(x)N_(y)) as shown in FIG. 5.

Next, the flexible structures in the form of cantilevers 26, 28 areformed in the wafer 70 using known fabrication procedures, for examplethe fabrication procedure for cantilever beams outlined in the paper LW. Rangelow et al, Proc. SPIE 2879, 56 (1996). This results in astructure such as is shown in FIG. 6. In actual fact, FIG. 6 has had thefirst part of the wafer in front of the plane of the drawing removed andin practice a fourth cantilever corresponding to the one 26 at the topof the drawing of FIG. 6 is provided symmetrically on the other side ofthe template 16 (which is itself sectioned in the drawing). Asmentioned, the fabrication of the cantilevers is described in the abovenamed paper by L W. Rangelow et al. It is also described in the Germanpatent application DE 103 03 040 A1 and in the article first referencedin this application (J. Vac. Sci. Technol. B 21(6) November/December2003, pages 3102 to 3107). As can be seen from FIG. 6, the flexiblestructures here have the form of cantilevers 26, 28 and 28′ which arebasically made in the silicon layer 74 of the wafer 70 and which areonly fixed to the wafer at one end. The tips 30, 32 and 32′ can beintegrated into the silicon material and formed at the beginning of themicro-machining used to form the cantilevers, or can be electron-beamdeposited tips made after the fabrication of the cantilever beam andgrown directly on an aluminium micro-heater which is incorporated intothe cantilever structure as part of the bimorph actuator used to adjustthe vertical position of the tip of the cantilever. With thisarrangement of the cantilevers four surface relief markings wouldtypically be provided, one at the middle of each side of a squareimprint area 20.

In one example the silicon wafer used is a silicon-on-insulator wafer of3″ diameter comprising a 40 μm thick Si-layer 74 (12 Ohmcm n-typesilicon <100> orientation) bonded to a 70 nm thick thermal oxide layergrown on <100> base silicon wafer. The oxide layer 72 is used as anetching stop layer. If the silicon tip has to be integrated on thecantilever, a thermal oxide needs to be grown and patterned to form an8000 Å thick mask which is subsequently used for wet etching of the Sitip. After a standard RCA clean, an 8000 Å thick oxide is grown. Thisfilm is patterned and the resist mask over the oxide is employed as amask for the boron contact implantation at 1.1×10¹⁵ cm², 30 keV. Thisresist mask is then removed using m-wave plasma stripping and this isfollowed by growth of passivating thermal oxide during a 1 h anneal at900° C. Using a resist mask again, the piezoresistors incorporated inthe sensor system associated with the cantilevers are configured in aWheatstone bridge configuration defined in the oxide layer and boronimplanted at 4×10¹⁴ cm² 20 keV, followed by growth of passivatingthermal oxide during annealing at 1050° C. for 30 minutes. Thecantilevers are then patterned and plasma-etched to open contact holesto the highly doped areas. Aluminium for the contacts to thepiezoresistors in the metal layer forming the micro-heater and bimorphactuators is then deposited and annealed in a forming gas at 410° C. for50 minutes. The oxide layer on the back of the wafer is patterned and agas chopping reacted ion etching process (GChRIE) combined with KOH wetstep is used to release the cantilever membranes and partially dice thewafer. The buried oxide used to stop the silicon etch is then removedwith a buffered oxide etch solution, using a mechanical wafer chuck toprotect the topside of the cantilevers. To form the cantilever beam andto cut up the single sensor chip employing GChRIE step, a thick resistmask is used. Finally, the resist mask is removed in oxygen plasma.

Thus, in FIG. 6, the template 16 is integrally incorporated on the waferwith the flexible structures in the form of cantilevers 26, 28, 28′. Itis, however, also possible to form the template 16 separately and toincorporate it into the wafer 70 by forming a suitable recess in thewafer and bonding the template in place using a convenient means, suchas, for example, an adhesive. This is shown in FIG. 7 where the separatetemplate 16 is received in a well 82 of the wafer 70.

Another possibility which builds on the construction of FIG. 7 is shownin FIG. 8. Here, the wafer is removed at 84 at the base of the well 82and this has the advantage that an optical system can be used to viewthe first article through the transparent template 16 for basicalignment purposes and also for the purpose of exposing the photocurablelayer during nano-imprint lithography.

One possibility for detecting the contact of the flexible structure,i.e. the tip thereof, with the edges of the alignment mark is shown inFIG. 9. This embodiment uses optical detection of the deflection of theflexible structures 26, 28 due to the interaction with the surfacerelief markings 22, 24 on the first article. This system is illustratedin FIG. 9. A reflective layer 86 is provided on the surface of thecantilevers 26, 28 and respective optical beams 88 are used to measurethe deflection of the cantilevers, which changes the angle of reflectionof the respective reflected rays 88′. A suitable detector, for example aposition sensitive photosensor or a finely resolving linear photodiodearray 90, can be used to measure the angle of reflection of the lightreflected at the reflected layer on the cantilevers. This provides avery accurate measurement because the angular deflection of thecantilever is effectively amplified by the movement of the reflectedlight beam. Reference should also be made here to FIGS. 22 to 24 whichfurther illustrate the detection of the edges of an alignment mark suchas 22 or 24 by moving or “dragging” a flexible structure, here with afree end forming a tip 30 or 32 rather than a pointed formationprojecting from an otherwise flat arm, which is not strictly speakingnecessary, across the alignment mark in accordance with the arrow Pshown in FIG. 23. FIG. 22 shows how the beam of light 88 from a laserstrikes the flexible structure, in this case at a position away from theend of the flexible structure intermediate its free end and its morerigidly mounted end 202, where it adjoins the wafer carrying thetemplate or a mounting base used to attach it to the wafer carrying thetemplate.

As can be seen from FIGS. 23 and 24 the bending deflection of theflexible structure changes each time the flexible structure moves overan edge of the alignment mark or into contact with a base portionthereof between two raised bars and this changes the angle of reflectionof the reflected light beam 88′ (FIG. 24) so that the signal 204 fromthe position sensitive detector changes as illustrated in the inset 206of FIG. 22. the edge positions can be seen quite clearly in the detectorsignal and their spacing in time accurately reflects the physicalspacing of the edges at the speed of scanning movement of the flexiblestructure. From this the relative position of the alignment mark can befound accurately An absolute measurement of the position of the edges isnot essential, the shape of the signal can itself be used to determinethe relative alignment, this can be done very fast. It will be notedthat FIG. 23 shows two different positions of the flexible structure,one in dotted lines and one as solid line. FIG. 24 shows thecorresponding reflected light beam 88′ again once in dotted lines andonce in a solid line.

FIG. 10 shows an arrangement similar to FIG. 9 but in this case thetemplate 16 and the cantilevers 26, 28 are formed separately from thewafer 70 and subsequently joined to it by adhesive bonding. FIG. 10 alsoshows the use of an integrated piezoactuator on the cantilever beam tomove the cantilever beam in the z direction.

FIG. 11 shows a plan view of the surface of a cantilever 28 such as isillustrated schematically at three positions in each of FIGS. 6, 7 and8, however with the difference that the contacts 120, 122, 124 and 126and the associated leads 121, 123, 125 and 127 are now provided on theother side of the wafer 14, i.e. at the top in FIG. 11 and in FIGS. 12Aand 12B corresponding to the bottom of the wafer in FIGS. 6, 7 and 8.This is the reason the leads are shown in broken lines as are theoutlines of the contacts. Two of the four electrical contacts 120, 122,124 and 126, more specifically the contacts 120 and 126 are connected toa bimorph actuator 128 via respective leads 121 and 127. The other twocontacts 122 and 124 are connected to a Wheatstone bridge circuitcomprising four piezoresistors (not shown in FIG. 11) forming apiezoresistive sensor 130 via respective leads 123 and 125.

The design of the bimorph actuator 128 and the piezoresistors can beseen in more detail from FIGS. 12A and 12B. The bimorph actuator isformed by the Si layer 74 of the cantilever 28, by an additionaloverlaid layer 132 of Si02 and by an Al layer 134 provided on top of theSi02 layer 132. The aluminium layer 134 is patterned to form two leadsleading via respective via-holes in the wafer 14 to the contacts 120 and126 which are provided on the top surface of the silicon wafer 14 inFIGS. 12A and 12B but not shown in the section plane of FIGS. 12A and12B. Although the via-holes associated with the leads 121 and 127 to thecontacts 120 and 126 are not shown in FIGS. 12A and 12B these figures doshow a via-hole 136 associated with a lead 125 to the contact 124associated with the Wheatstone bridge detection circuit and thepiezoresistor 138. The lead 123 to the contact 122 is also not shown inFIGS. 12A and 12B.

As described in the above referenced article (J. Vac. Sci. Technol. B21(6) November/December 2003, pages 3102 to 3107) the three layers 74,132 and 134 forth a bimetallic (Si 74/Al 134) structure which can bedeflected by applying power from a suitable power supply via thecontacts 120 and 126 to the aluminium layer 134. More specifically thecantilever with the integrated bimorph actuator is normally bent to orbeyond the advanced position shown in FIG. 12A due to residual stresscreated during the deposition of the bimorph actuator.

DC heating power applied to the aluminium layer can be used to causedifferential expansion of the bimetallic structure so that thecantilever can be retracted into the position shown in FIG. 12B in whichthe tip 32 lies behind the front face of the template 16 permitting theprinting operation in which the template 16 is used to pattern a layerof imprint resist 18 deposited on the imprint area 20 of the first wafer10.

The sensing operation, by which the tip 32 of the cantilever is used inan advanced position in which it projects beyond the front face of thetemplate 16 (the lower side of the template in FIGS. 12A and 12B), totouch and sense the surface relief marking 22, without the templatecontacting the first article, can either be a natural advanced positionof the cantilever or an advanced position controlled by the supply of dcheating power to the aluminium layer 134. Thus the cantilever tip 32 isjust beyond the front face of the template 16 and is able to touch thesurface of the first article 10 in accordance with the preferred sensingmode of the invention.

The Al layer 134, the Si02 layer 132 and the Si layer 75 can bepatterned not just for the formation of the via-holes such as 136, whichare lined by the SiO2 layer 132 and the Al layer 134 but also to definefour piezoresistors such as 138 (only one shown) and the Al leadsconnecting them together in the Wheatstone bridge configuration and thecontacts 122 and 124 via the respective leads such as 125 passingthrough respective via holes such as 136.

Each piezoresistor comprises a p+ boron doped portion of the Si layerextending between two p++ boron doped electrodes 144 and 146 contactedby Al contacts 140 and 142 formed by regions of the Al layer 134. Theprecise patterning is selected so that the piezoresistors areelectrically separated from the bimorph actuator and the associatedleads and contacts.

Turning now to FIGS. 13A and 13B there can be seen an alternative typeof flexible structure in the form of a bridge. It can be seen that thebridge 92 carrying the tip 30 is supported at both ends 91, 93 in thesilicon wafer 70, and that a cavity 94 is formed on both sides of thebridge and underneath the bridge to allow for deflection of the tip ofthe cantilever in the z direction.

FIG. 14A shows another alternative form of flexible structure, here inthe form of a membrane 96 supported at three points 95, 97, 99 on thesilicon wafer 70. The sections of FIGS. 14B and 14C show that thesilicon material around the flexible membrane is removed to allow it todeflect in the z direction as is required to determine interaction withthe first article. I.e. the membrane again sits in a cavity 94.

FIG. 15 shows a membrane 98 which is supported on the silicon structureat four points 101, 103, 105 and 107 and again it can be seen from thesections 15B and 15C that the silicon material is removed around theflexible membrane to permit deflection in the z direction. SectionXVD-XVD is the same as the section XVC-XVC shown in FIG. 15C.

Finally, FIG. 16 shows a use of the present invention in a field otherthan that of semiconductor manufacture. In FIG. 16 the first article 10takes the form of a wafer with a plurality of recesses 110 formedtherein into which a chemical or biological reagent or sample can beintroduced. The reference numeral 14 shows a second article, which isanother wafer having an array of points 112 provided on it in a patterncomplementary to those of the recesses of the first article 10. Achemical or biological sample or reagent can be provided on each of thepoints. If the second article 14 is then inverted and placed on top ofthe first article 10, then, if the alignment is correct, the tips of thepoints 112 on the second article 14 enter into the recesses 110 in thefirst article 10 and the biological or chemical reaction that is beinginvestigated can take place. It will be appreciated that the surfacerelief markings and the flexible structures described in detail abovewill be respectively provided on the first article 10 and on the secondarticle 14 to achieve the desired precise alignment. A similar techniquecan be used to align a detector for readout of the reaction. I.e. thedetector would form a further first or second article, depending onwhere the readout is to be effected, i.e. at the second article or thefirst article. This detector could also be integrated into the first orsecond article.

In this specification the same reference numerals have been used inseveral of the drawings and the description given in connection with onedrawing will be understood to apply to the items marked with the samereference numeral in other drawings unless something is stated to thecontrary.

Turning now to FIGS. 17 to 21 some further examples will be given forways in which a flexible structure can be used in a touching mode toquickly retrieve signal information relating to contact of the flexiblestructure 26 or 28 with edges of an alignment mark 22 or 24.

In FIG. 17 a piezoresistor 208 is formed on the flexible structureadjacent its base 202 and bending deflection of the flexible structureleads to strain at its bas 202 which results in a change in theresistance of the piezoresistor. This is detected by the voltage dropwhich occurs when a voltage is applied across the piezoresistor from twopads 210 and 212 provided on the wafer which are connected to thepiezoresistor vial appropriate leads 214, 216. Again the shape of thevoltage drop reflects the positions of the edges of the alignment mark.It will be noted that these edges are not uniformly spaced and thisfacilitates easy recognition of the precise relative position of thealignment mark.

The arrangement of FIG. 18 is rather similar to that of FIG. 17 exceptthat here the piezoreistor is replaced by a piezoelectric layer. Ondeflection of the flexible structure 26 or 28 at the edges of thealignment mark 22 or 24 the voltage across the piezoelectric layer,which can be picked up, i.e. measured, at the electrodes 210 and 212,changes as a function of the change in strain resulting from the bendingdeflection. Again the position of the edges of the alignment mark can bedetermined from the voltage signal 204.

The arrangement of FIG. 19 is again rather similar to those of FIGS. 17and 18, except that here a varying voltage signal 204 is detected as aresult of a change in capacitance resulting from the bending deflectionof the flexible structure as it flexible structure 26 or 28 travelsacross the edges of the alignment mark 22 or 24 during scanning movementin the direction P. This bending deflection of the flexible structure,which forms one plate or electrode 213 of the capacitor, results in achange of its distance from a conductive element 215 forming the secondplate or electrode of the capacitor which is spaced from the firstelectrode or plate by an insulator 217.

In FIG. 20 two coils 220 and 222 are realised on the flexible structureone at its base 202 and the other 222 on the wafer in close proximity tothe first coil 220. A voltage applied to the first coil 220 via itsrespective electrodes 224 and 226 and leads 228 and 230 induces achanging voltage in the second coil due to the relative movement withchanging bending deflection of the flexible structure and results in theinduced voltage signal 204 in the inset 206 as the flexible structure 26or 28 is moved relative to the edges of the alignment mark 22 or 24 inthe direction of the arrow P. This induced voltage is picked up via theleads 232 and 234 at the electrodes 236 and 238 on the wafer forming thetemplate.

In FIG. 21 an interdigital structure 240 similar to a SAW device ispresent on the flexible structure in the vicinity of its base 202 andbending deflections of the flexible structure 26 or 28 as it is moved inthe direction of the arrow P with changing deflection at the edges ofthe alignment mark results in a surface wave transition signal which canbe picked up at the electrical contacts 210 and 212 via the leads 214and 216. again the shape of the surface wave transition signal providesinformation on the positions of the edges of the alignment mark 22 or24.

A discussion of various known methods for forming semiconductor circuitsusing printing techniques and of various other concepts useful for anunderstanding of the invention will now be given in the followingappendix. It will be appreciated that the present invention can be usedin conjunction with all these methods to achieve high alignmentaccuracy.

Appendix and discussion of state of the art practices:

The technology is typically referred to as Step and Flash ImprintLithography (S-FIL). For the further development of nanoimprintlithography the related overlay problems must be solved if this type oftechnology is to be applied to high-density silicon integratedcircuitry. Nanoimprint lithography is generally understood to cover aclass of new methods for the replication of nanometer-scale patternsdown to 10 nm on solid materials.

Three different varieties of nanoimprint lithography will now bediscussed. Soft lithography generally refers to the process oftransferring a self-assembled monolayer using a flexible template asdescribed in the paper by Whitesides et al. Y. Xia and G. M. Whitesides,Angew. Chem., Int. Ed. Engl. 37, 550 (1998). These authors formed atemplate by applying a liquid precursor to polydimethylsiloxane over amaster mask produced using either electron beam or optical lithography.

A second process known as first nanoimprint lithography (NIL), developedby Chou et al is described in the paper by S. Y. Chou, P. R. Krauss, andP. J. Renstrom, J. Vac. Sci. Technol. B 14, 4129 (1996). These authorsuse a solid mold, such as silicon or nickel. The imprint process isaccomplished by heating a resist above its glass transition temperatureand imparting a relatively large force to transfer the image into theheated resist.

A derivative of NIL, ultraviolet nanoimprint lithography (or UV-NIL)addresses the issue of alignment by using a transparent template,thereby facilitating conventional optically aligned overlay techniques.The use of a quartz template enables the photocuring process to occurand also opened up the potential for optical alignment of the wafer andthe template. In addition, the imprint process is performed at lowpressures and at room temperature, which minimizes magnification anddistortion errors. In this connection reference is made to the papers byM. Otto, M. Bender, B. Hadam, B. Spangenberg, and H. Kurz,Microelectron. Eng. 57, 361 (2001) and M. Colburn, S. Johnson, M.Stewart, S. Damle, T. Bailey, B. Choi, M. Wedlake, T. Michaelson, S. V.Sreenivasan, J. Ekerdt, and C. G. Wilson, Proc. SPIE 379 (1999).

It is important to note that nanoimprint lithography is still at thestart of its development, there are several companies that are nowoffering cosmetic imprint tools. In addition to Molecular Imprints Inc.(USA), Electronic Visions Group (Austria), Nanonex (U.S.), Obducat(Sweden), and Suss Microtec (Germany) have systems ready for purchase.

The Step and Flash Imprint Lithography (S-FIL™) technology referred toabove was developed at the University of Texas at Austin. The techniqueis based on the ancient craft of embossing, with an adaptation to modernsemiconductor needs. The technique uses a fused silica template with acircuit pattern etched into it. A commercialized version of an S-FILtool is now available from Molecular Imprints Inc.

The fused silica surface, covered with a release layer, is gentlypressed into a thin layer of low viscosity, silicon-containing monomer.When illuminated by a UV lamp, the surface is polymerized into a hardmaterial. Upon separation of the fused silica template, the circuitpattern is left on the surface. A residual layer of polymer betweenfeatures is eliminated by an etch process, and a perfect replica of thepattern is ready to be used in semiconductor processing for etch ordeposition. Only the template fabrication process, typicallyaccomplished with an e-beam writer, limits the resolution of thefeatures. Features as small as 20 nm have been made to date that exceedthe present requirements of the ITRS (International Technology RoadmapSemiconductors).

S-FIL has several important advantages over conventional opticallithography and EUV lithography. The parameters in the classicphotolithography resolution formula (kl, NA, and lambda) are notrelevant to S-Fit because the technology does not use reduction lenses.Investigations, by Molecular Imprints Inc. and others, in the sub-100 nmregime indicate that the resolution is only limited by the patternresolution on the template. The resolution of S-FIL is a direct functionof the resolution of the template fabricating process. Therefore, theS-FIL tools are multi-generational and should have a longer life ascompared to optical lithography tools that have to be replaced when theexposure wavelength is decreased (decreasing the wavelength increasesthe optical resolution, i.e. reduces the size of features which can berealised). S-FIL templates are typically fabricated using conventionaloptical phase-shift mask technology. Electron beam writers that providehigh resolution (below 10 nm), but lack the throughput required for massproduction, are used. S-FIL lithography therefore takes advantage ofresolution offered by e-beam technology without compromising throughputand tool life.

S-FIL™ is a bi-layer approach using a low viscosity, UV-curable imprintsolution deposited on an underlying organic planarization layer. Thetemplate is rigid and transparent, allowing for ITV curing of theimprint solution.

With S-FIL, an organic planarization layer is spin-coated on a siliconsubstrate. Then a low viscosity, photopolymerizable imprint solution isdispensed in droplets on the wafer to form an etch barrier in theimprint area. The template is then lowered into liquid-contact with thesubstrate, displacing the solution, filling the imprint field, andtrapping the photopolymerizable imprint solution in the template relief.Irradiation with UV light through the back side of the template curesthe solution. The template separates from the substrate, leaving anorgano-silicon relief image that is an exact replica of the templatepattern. A short halogen etch is used to clear undisplaced, curedimprint solution. A subsequent oxygen reactive ion etch into theplanarization layer amplifies the aspect ratio of the imprinted image.

The S-FIL™ template and substrate, which are typically less than 250nanometers apart, are in liquid contact due to the low viscosity imprintsolution, which also behaves as a lubricant. This facilitates fineadjustment of the wafer and template. Although workers in this field areconfident that they can demonstrate alignment capabilities that rivalconventional state of the art lithography systems what this means isoptical alignment within about 100 nm.

Molecular Imprints Inc. describes imprint lithography as a 1x-patterntransfer process. The design and production of a high-quality templateis therefore a key factor for its success. Currently, templates areprepared following standard phase-shift mask manufacturing techniques: Aresist-on-chromium-coated quartz mask blank is patterned with anelectron beam, and the exposed resist is developed away (i.e., apositive tone process). Then, the exposed chromium is removed with a dryetch process and the quartz is etched using a standard phase-shift etchprocess, creating topography in mask quartz.

The S-FIL™ technique from Molecular Imprints Inc. uses a standard6-inch×6-inch×0.250-inch fused silica blank. During photomask processingthe chrome is removed leaving only the circuit pattern etched into it.The photomask is divided into four quadrants and the pattern isgenerated (can be one or more layers). The scheme enables die to dieinspection, improving ease of manufacturing. The final template istypically sized to a 65×65 mm size. This process is described in thefollowing papers and articles:

“High resolution templates for step and flash imprint lithography” D. J.Resnick JM3 1(3) 284-289 (October 2002) and

“Analysis of critical dimension uniformity for step and flash imprintlithography” David P. Mancini, Physical Sciences Research Laboratories,Motorola Labs, Tempe, AZ USA

1. Apparatus for aligning a first article with a second article, saidfirst article having at least one surface relief marking and said secondarticle having at least one flexible structure having a sensing tip;means adapted for positioning said second article adjacent to said firstarticle with a provision for at least restricted relative movementbetween said first and second articles; a detector for detectingdeflection of said at least one flexible structure due to an interactionwith said at least one surface relief marking and for providingdetection signals related to said interaction; a memory containingstored information related to a topology of said surface relief marking;means for comparing said detection signals with the stored informationto generate position control signals relating to a desired alignment;and steering means for steering a movement of at least one of said firstand second articles relative to the other to achieve said desiredalignment: wherein said second article includes a template and whereinsaid detector is adapted to detect deflection of said flexible structureby the sensing tip of the flexible structure touching the surface,wherein an actuator is integrated with the flexible structure to permitdeflection of the flexible structure between at least a first advancedposition in which the sensing tip is disposed in front of a front faceof said template and a second retracted position in which its thesensing tip is disposed behind said front face of said template.
 2. Anapparatus in accordance with claim 1, said steering means being selectedfrom the group consisting of piezoelectric actuators, thermal actuators,electromagnetic actuators, and oscillatory actuators.
 3. An apparatus inaccordance with claim 1, there being a plurality of surface reliefmarkings on said first article and a plurality of flexible structuresprovided on said second article (14), there being the same number offlexible structures as there are surface relief markings.
 4. Anapparatus in accordance with claim 1, wherein the at least one flexiblestructure is selected from the group consisting of: a cantilever, aflexible bridge supported at first and second points, and a flexiblemembrane supported at a plurality of points.
 5. An apparatus inaccordance with claim 1, wherein each surface relief marking isintegrally formed with said first article and is one of a naturalfeature of said first article and an artificial feature of said firstarticle.
 6. An apparatus in accordance with claim 1, wherein saidflexible structure is either: integrally formed with said second articleor is distinct from said second article, but physically connected tosaid second article.
 7. An apparatus in accordance with claim 1, whereinsaid first article is one of: a conductive substrate, a semiconductorwafer, an insulating substrate, a glass, metal or plastic article, abiological sample carrier, and a chemical sample carrier.
 8. Anapparatus in accordance with claim 1, wherein said template is one of atemplate for nanoimprint photography, a biological sample dispenser, achemical sample dispenser, a biological sample readout device, and achemical sample readout device.
 9. An apparatus in accordance with claim1, wherein the detector is incorporated on the flexible structure. 10.An apparatus in accordance with claim 1 wherein the detector is selectedfrom the group consisting of: a reflecting surface provided on saidflexible structure and an associated optical detection system formeasuring said deflection at said reflecting surface, a piezoresistivestructure incorporated at said flexible structure, a piezoelectricstructure incorporated at said flexible structure, a capacitive detectorfor detecting deflection of said flexible structure, an inductivedetector for detecting deflection of said flexible structure, and asurface acoustic wave structure incorporated at said flexible structure.11. An apparatus in accordance with claim 1, wherein said flexiblestructure is provided as a bimorph structure.
 12. Apparatus for aligninga first article with a second article, said first article having atleast one surface relief marking and said second article having at leastone flexible structure having a sensing tip; means adapted forpositioning said second article adjacent to the said first article withmeans for generating at least restricted relative movement between saidfirst and second articles; a detector for detecting deflection of saidat least one flexible structure due to an interaction with said at leastone surface relief marking and for providing detection signals relatedto said interaction; a memory containing stored information related to atopology of said surface relief marking; means for comparing saiddetection signals with the stored information to generate positioncontrol signals relating to a desired alignment and; steering means forsteering a movement of at least one of said first and second articlesrelative to the other to achieve said desired alignment; wherein saidsecond article includes a template, wherein said detector is adapted todetect deflection of said flexible structure by the sensing tip of theflexible structure touching the surface, wherein an actuator isintegrated with the flexible structure to permit deflection of theflexible structure between at least a first advanced position in whichthe sensing tip is disposed in front of a front face of said templateand a second retracted position in which the sensing tip is disposedbehind said front face of said template.